On December 14 the US State Department announced sanctions against the Turkish defence industry, after Ankara's purchase of the Russian S-400 anti-aircraft system.
The move was based on a 2017 piece of legislation, the Countering America's Adversaries through Sanctions Act, penalising Turkey for finalising the $2.5 billion arms deal.
Secretary of State Mike Pompeo said that Ankara had "knowingly engaged in a significant transaction with Rosoboronexport, Russia’s main arms export entity, by procuring the S-400 surface-to-air missile system".
But why has this system – supposedly cutting-edge technology, created so much consternation in Washington? Here is some background on the S-400, what its makers claim it is capable of, and why it might not be as potent as some claim.
What is the S-400 capable of?
A mobile air-defence missile battery, it is equipped with a powerful radar system which can track up to 80 targets simultaneously, engaging them with high-velocity, high-altitude missiles at very long ranges. The radars with the system also operate together on different frequencies, allowing for more accurate detection of enemy targets.
With this array of capabilities, the S-400 ensures that attacking aircraft will struggle to get close to enemy airspace, let alone successfully carry out a bombing raid. Its most recent missiles, including the 40N6, can engage targets at altitudes of 24,000 metres, up to 400 kilometres away – or so the makers claim.
More concerning for Nato, the long range of the S-400 and its predecessor the S-300 is so great that while technically a defensive weapon, it can engage targets over foreign territory, potentially an offensive capability.
Why is the US so worried by the S-400?
Initially, Washington was concerned that Turkey could test the S-400's powerful radar on F-35 jets, which the Turks intended to buy and manufacture domestically. This was a red line for Washington, which closely guards the F-35s stealth technology and complex avionics. US exports of F-35s to Turkey were subsequently cancelled before the first aircraft arrived.
But Washington is no doubt concerned for its Greek allies —also in Nato —who operate older aircraft than the stealthy F-35: the current Greek aircraft inventory could be vulnerable to the S-400.
So the S-400 represents an unstoppable threat to the US, and this is why they are concerned?
This depends on who you ask. For one thing, the version sold to Turkey may not have the same features as S-400s in Russian inventories.
"The S-400 system that Turkey has purchased is likely to lack some of the more advanced software and radar features compared to the Russian Armed Forces version - exports are usually downgraded, let alone when given to a Nato member state," according to Justin Bronk, a research fellow at the Royal United Services Institute in London.
Nato was able to study an S-300 after Cyprus bought the system from Russia at the end of the 1990s. In what is arguably a historical irony, Turkey threatened to attack Cyprus unless the order was cancelled.
The crisis was finally defused when the system was transferred to Greece, allowing Nato to examine the system – albeit a predecessor of the S-400. In 2014, the Greek air force even test fired its ageing S-300. Since the Cyprus S-300 crisis, Israel, the US and other Nato countries have made herculean efforts to devise countermeasures to defeat the S-300 (used by Iran and Syria) and the S-400 system.
Aircraft such as the F-35 and weapons like the Joint Air-to-Surface Standoff Missile (JASSM) were developed, the latter being a cruise missile that uses stealth technology, potentially evading the S-400’s powerful radar.
Nineteen JASSM missiles were fired at the Syrian regime in the spring of 2018, in response to a Syrian chemical weapons attack that killed civilians. At the time, Russia had deployed S-300 and S-400 systems stationed in Syria. The use of the JASSM therefore, was in anticipation that the Russians and Syrians would try to shoot down some of the older cruise missiles used by the Americans, using the S-400 and S-300.
So the system has limitations?
While the S-400 could be lethal against a badly planned attack, its maximum range may depend on what other equipment its users decide to deploy in tandem with the system. This is because of the curvature of the Earth, which limits radar projection to the horizon. Very low-flying objects, such as cruise missiles or even aircraft, may therefore not be detectable until they are close enough to attack the system. At low level, this range could be as little as 40km.
For the S-400 to reach its full potential, it needs to be integrated with a large number of other radar systems, including airborne radar such as airborne early warning and control planes, or Awac planes, supplemented by smaller air defence systems. This is something the Russians have invested a lot of time and money in — an "integrated air defence" system, or IADs. Turkey may have a long way to go before having such capability.
"A solitary S-400 system, while very capable against a wide range of aerial and ballistic targets, is far less capable than a system integrated within a wider layered IADS," according to Mr Bronk.
Mr Bronk says that Turkey's promise to keep the S-400 separate from Nato's air defence network will critically limit the system, denying it the level of integration enjoyed by S-400s in Russian service.
"The S-400 in Turkish service will be far less capable than Russian equivalents working as a core lynchpin of a broader IADS," he says.
But new technology is also in service that could pose a serious threat to the S-400 on the battlefield, according to Mr Bronk.
"Loitering munitions pose a serious threat if launch platforms can get close enough to release significant numbers of them, which can overwhelm the self-defence capabilities of the system," he says, referring to munitions that were commonly referred to as "suicide drones."